Topic 16.4C, 16.4E–16.4F · Core and Supplement
Fertilisation, Implantation and Pregnancy
This page follows one continuous sequence — fertilisation, zygote, embryo, implantation, then the structures that keep a fetus alive — and the sequence matters as much as any individual fact. Implantation is not fertilisation happening again; it's a separate, later event, in a separate location.
Fertilisation in humans — Core
Human fertilisation is the fusion of the nucleus of a sperm cell with the nucleus of an egg cell, forming a zygote. It usually occurs in an oviduct. “Sperm meets egg” is too vague on its own — the biological event Cambridge is actually testing is nuclear fusion specifically, not simply the two cells coming into contact.
From zygote to implantation — Core
Once fertilisation forms a zygote, that single cell divides repeatedly, developing into what the syllabus describes as a ball of cells — the embryo. The embryo then travels to the uterus and implants into the lining there. Implantation is a separate, later step from fertilisation, and it happens in a different location: fertilisation in the oviduct, implantation in the uterus. Keep zygote, embryo and fetus as three distinct developmental stages too — the zygote is the very first cell, the embryo the early multicellular stage, and the fetus the later stage of development.
Structures that support the fetus — Core
| Structure | Function |
|---|---|
| Placenta | Forms the exchange interface between the mother’s blood and the fetus’s blood |
| Umbilical cord | Connects the fetus to the placenta and carries fetal blood between them |
| Amniotic sac | The membrane that surrounds and encloses the developing fetus |
| Amniotic fluid | Cushions the fetus against mechanical shock |
Keep the sac and the fluid it holds as two separate ideas with two separate jobs: the amniotic sac is the membrane that contains the fluid, but it’s specifically the amniotic fluid inside it that actually does the cushioning. Crediting the sac itself with the cushioning function is a common but avoidable slip.
Placental exchange — Supplement
The mother and fetus keep entirely separate blood circulations throughout pregnancy — their blood never mixes directly. What crosses between them does so across the placenta, in two directions. From mother to fetus: oxygen and dissolved nutrients such as glucose. From fetus to mother: carbon dioxide and excretory products such as urea. The umbilical cord is simply the transport route for fetal blood to and from that exchange surface.
“The placenta feeds the baby” is the kind of answer that sounds right but isn’t precise enough for a mark — the stronger version states the actual substances and their direction: dissolved nutrients and oxygen pass from maternal blood to fetal blood, while carbon dioxide and excretory products pass the other way.
It’s also worth being clear that the placenta is not a complete barrier. Some pathogens and some toxins can cross it and affect the developing fetus, so “the placenta stops harmful substances reaching the baby” is biologically too absolute a claim — its role is exchange, not filtration, and that exchange surface doesn’t distinguish perfectly between what’s helpful and what isn’t.
Where this leads next
Pregnancy itself depends on a hormone system that’s already been running since before fertilisation — covered from sex hormones in humans through to the full menstrual cycle and its hormonal control, including how that cycle is suppressed once pregnancy actually begins.